Foamed bodies of generally employed resins such as polyethylene, polypropylene and polystyrene resins have been conventionally used in various fields because of their light-weight properties, good heat insulating and cushioning properties. Since these foamed bodies of generally employed resins are hardly decomposed when allowed to stand in natural environment after uses, a number of techniques of recovery and recycling have been recently realized.
Recently, interest in global environment is increasing and environmental issues such as depletion of petroleum resources attract a lot of attention. In such circumstances, polylactic acid resins which are environmentally low-load plastics are now drawn much attention in place of conventional plastics derived from petroleum resources. Polylactic acid resins are capable of being produced from plant such as corn as a raw material and decomposed into carbon dioxide and water, when left in the natural environment. Therefore, such polylactic acid resins are inferred to be used more generally. Foamed bodies made from polylactic acid resins are being developed in the same way as that of conventional general-use resins.
However, a known polylactic acid resin foamed sheet (Patent Document 1) uses a non-crystalline polylactic acid resin and, therefore, has a problem in heat resistance though it has good moldability. In particular, the foamed sheet is deformed even at room temperature. A crystalline polylactic acid, on the other hand, has good heat resistance but has a problem in foamability and thermoformability. With a crystalline polylactic acid, therefore, it is difficult to produce sheet-like or plate-like foamed bodies having the same functions as those of the conventionally used sheet-like or plate-like polystyrene resin foamed bodies. Even when foamed sheets are prepared, crystalline polylactic acid resin foamed sheets (Patent Documents 2 and 3) have poor thermoformability. Further, it is not easy to thermoform such sheets also because of a high apparent density, non-uniform cell shapes and a low closed cell content. Even when molded articles are obtained, there is a room to improve mechanical strength such as tensile strength and compressive strength.
Patent Document 1: Japanese Unexamined Patent Publication No. JP-A-2002-322309 (claims)
Patent Document 2: Japanese Unexamined Patent Publication No. JP-A-2002-3709 (claims)
Patent Document 3: Japanese Unexamined Patent Publication No. JP-A-2000-136259 (claims)